Case 9 Anatomy Flashcards
what is the level of the hyoid bone?
C3
what is the level of the lower border of the cricoid cartilage?
C6
what is the level of the thyroid notch?
C4
what is the level of the suprasternal notch?
T2/3
what is the thyroid gland made up of?
an isthmus and two lateral lobes
- in some cases, a pyramidal lobe is also present
where does apex and base of lateral lobe extend to?
- apex extends up to the oblique lines of the thyroid cartilage
- base to the level of the fourth or fifth tracheal ring
which tracheal rings does the isthmus cross anterior to?
2nd, 3rd and 4th
where does the pyramidal lobe project upwards from and where to?
from the isthmus to the left of the midline
what is the thyroid gland enclosed by?
a thin connective tissue capsule, from which septa project into the gland
what surrounds the glad and the capsule (thin connective tissue)?
a loose connective tissue sheath
what is this sheath derived from?
the pretracheal fascia (one of layers of cervical fascia)
what are the anterolateral relations of the thyroid gland?
- Anterior border of sternocleidomastoid
- Sternohyoid muscle
- Superior belly of omohyoid
- Sternothyroid muscle
what are the posterolateral relations of the thyroid gland?
- Vagus nerve
- Common carotid artery
- Carotid sheath
- Internal jugular vein
what are the medial relations of the thyroid gland?
- Cricothyroid muscle
- Recurrent laryngeal nerve
- Larynx
- Trachea
- Oesophagus
- External laryngeal nerve
- Pharynx
what are the anterior relations of the isthmus of the thyroid gland?
- Sternothyroid muscle
- Anterior jugular vein (blood vessel formed by the confluence of the superficial submandibular veins)
- Sternohyoid muscle
what are the posterior relations of the isthmus of the thyroid gland?
• Second, third and fourth rings of tracheal cartilages
where do the blood vessels supplying the thyroid gland lie?
between the capsule and the fascial sheath
what is the arterial supply of the thyroid gland?
- Arterial blood is supplied by the superior and inferior thyroid arteries; the superior thyroid artery supplies the upper pole of each lobe whilst the inferior thyroid artery supplies the posteroinferior aspect
- There is considerable anastomosis between vessels
where does the superior thyroid artery arise from?
the external carotid artery
which nerve runs alongside the superior thyroid artery?
external branch of superior laryngeal nerve??
where does the inferior thyroid artery arise from?
the thyrocervical trunk
where does the thyrocervical trunk arise from?
the subclavian artery
(branch of the subclavian artery arising from the first portion of the vessel – it’s located distally to the vertebral artery and proximally to the costocervical trunk)
what crosses the inferior thyroid artery?
the recurrent laryngeal nerve
what percentage of the population have a third artery that contributes? what is this vessel? where does it arise from?
- In 10% of the population, a third artery also contributes
- This vessel is termed the thyroid ima artery and it may arise from the brachiocephalic trunk or arch of aorta, less commonly it has been seen to arise from the right common carotid, subclavian or internal thoracic arteries
what veins drain the thyroid gland and which parts?
Three pairs of veins, the superior, middle and inferior thyroid veins drain the superior, middle and inferior parts of the thyroid gland respectively
what do these veins form?
a plexus on the anterior aspect of the gland
what do the superior and middle thyroid veins empty into?
the internal jugular vein
what does the inferior thyroid vein empty into?
the brachiocephalic vein
where is the lymphatic network found?
A rich lymphatic network is found within the connective tissue septa of the thyroid gland
what do the lymphatic vessels drain into?
prelaryngeal, pretracheal and paratracheal nodes
where do efferent vessels from these nodes go into?
the deep cervical nodes
what type of innervation does the thyroid gland receive?
sympathetic
where do the post-ganglionic sympathetic fibres arise in? what do they pass through to reach the gland?
- superior, middle and inferior cervical ganglia
- pass through the cardiac, superior and inferior thyroid plexuses
what information is transmitted in these fibres?
vasomotor
what is thyroid goitre?
an enlargement of the thyroid gland which may occur due to over activity or underactivity of the gland
when the thyroid gland is enlarged, where does it spread? why?
- When the thyroid gland is enlarged it spreads anteriorly, posteriorly, inferiorly and laterally
- The thyroid gland is prevented from enlarging in a superior direction by the superior attachments of sternothyroid and sternohyoid muscles
what might a thyroid goitre compress?
oesophagus, recurrent laryngeal nerves and trachea
describe the development of the thyroid gland
- The thyroid gland develops from a thickening on the floor of the embryonic pharynx in the developing tongue
- A diverticulum develops from the thickening forming the thyroglossal duct
- The thyroglossal duct elongates and its distal end becomes bi-lobed and proliferates into the thyroid gland
- The thyroid gland descends through the neck anterior to the hyoid bone
- At seven weeks the gland reaches its adult position
- The thyroglossal duct subsequently degenerates but a pit remains on the dorsum of the tongue marking its site of origin = foramen caecum
ectopic thyroid glands
- when occur
- where located
- Ectopic thyroid glands occur when the gland fails to descend
- In incomplete descent the gland will be located high in the neck
- Rarely, the gland may fail to leave the tongue and a lingual thyroid develops
what percentage of population have pyramidal lobes present? what do they develop from?
- Pyramidal lobes are present in proximately 50% of the population
- These lobes extend upwards from the isthmus usually to the left of the median plane
- They develop from the remnants of the epithelium and connective tissue of the thyroglossal duct
where might accessory thyroid tissue present? when does it occur?
- Accessory thyroid tissue may present anywhere along the route of descent of the thyroglossal duct
- It occurs when isolated pockets of glandular tissue of the thyroglossal duct fails to degenerate
when do thyroglossal duct cysts occur? what might occur? where located?
- Thyroglossal duct cysts occur when the duct fails to degenerate leaving a persistent connection between the back of the tongue and the thyroid gland
- They can be asymptomatic but fluid may accumulate in small pockets and infection may occur
- The cysts are often located close to the hyoid bone
- Their position in the neck will rise if the patient sticks their tongue out
what do parathyroid glands secrete?
parathyroid hormone
how many parathyroid glands?
four:
- two superior
- two inferior
what shape and size is each parathyroid gland?
ovoid and approx. 6mm long
where are the parathyroid glands embedded? where do they lie in relation to the fascial sheath?
- embedded into the posterior aspect of the thyroid gland
- lie deep to the fascial sheath
which ones, the superior or inferior, have a more variable position?
inferior
where do the superior parathyroid glands usually lie in relation to where the superior thyroid vessels enter the thyroid gland?
below
where might the inferior parathyroid glands be located?
in the superior mediastinum
what are the parathyroid glands most commonly supplied blood by?
inferior thyroid artery
what are the parathyroid glands innervated by?
the postganglionic sympathetic fibres from the superior or middle cervical ganglia
what type of information do these nerves supply to the parathyroid glands?
vasomotor
(it is important to note that these nerves are vasomotor, not secretomotor – endocrine secretion of the parathyroid hormone is controlled hormonally)
what hormones does the thyroid gland secrete?
- thyroxine
- calcitonin
what does thyroxine do?
increases basal metabolic rate
what does calcitonin do?
reduces plasma calcium levels
what does parathyroid hormone do?
stimulates osteoclasts and increases plasma calcium levels
